Method for producing aluminum titanate sintered compact

ABSTRACT

The present invention provides a raw material composition for preparing a sintered body of aluminum titanate, the composition comprising (i) 100 parts by weight of a mixture comprising 40 to 50 mol % of TiO 2  and 60 to 50 mol % of Al 2 O 3 , (ii) 1 to 10 parts by weight of an alkali feldspar represented by the formula: (Na x K 1−x )AlSi 3 O 8  (0≦x≦1), and (iii) 1 to 10 parts by weight of at least one Mg-containing component selected from the group consisting of a Mg-containing oxide with spinel structure, MgCO 3  and MgO, and a process for preparing a sintered body of aluminum titanate comprising sintering a formed product prepared from the raw material composition at 1300 to 1700° C. According to the present invention, a sintered body of aluminum titanate having high mechanical strength and ability to be stably used at high temperatures, as well as its inherent properties of low coefficient of thermal expansion and high corrosion resistance, can be obtained.

TECHNICAL FIELD

The present invention relates to a raw material composition forpreparing a sintered body of aluminum titanate, a process for preparinga sintered body of aluminum titanate, and a sintered body of aluminumtitanate.

BACKGROUND ART

Sintered bodies of aluminum titanate have low coefficient of thermalexpansion and high corrosion resistance. They are known asheat-resistant materials which exhibit low wettability with slag,excellent corrosion resistance, spalling resistance and other excellentproperties when used as materials of, for example, containers, ladles,gutters, etc., for molten metals of aluminum, aluminum alloy, ferroalloyand the like. However, the sintered bodies of aluminum titanate, whosecrystal grains constituting the sintered bodies are anisotropic, tend tosuffer the following disadvantages: displacement at the crystal grainboundaries caused by stress due to the anisotropy of thermal expansioncoefficient when heated or cooled; and formation of micro cracks andapertures which may lead to lowered mechanical strength.

Hence, conventional sintered bodies of aluminum titanate haveinsufficient strength, and can not exhibit sufficient durabilityparticularly when high temperatures and heavy loads are applied thereto.

In addition, aluminum titanate is unstable at a temperature of 1280° C.or below. It tends to decompose into TiO₂ and Al₂O₃ when it is used in atemperature range of about 800 to 1280° C., and therefore is difficultto be used continuously in this temperature range.

To improve the sinterability of aluminum titanate and inhibit thermaldecomposition, additives such as silicon dioxide are mixed with rawmaterials prior to sintering. In this case, however, the refractorinessof the resulting sintered bodies tends to be lowered. For this reason,it has been impossible to obtain a sintered body of aluminum titanatethat has refractoriness so as to be usable at a temperature as high asabout 1400° C. or higher and also has high mechanical strength.

DISCLOSURE OF THE INVENTION

A primary object of the present invention is to provide a novel sinteredbody of aluminum titanate having mechanical strength improved to apractically usable level and ability to be stably used at hightemperatures as well as their inherent properties, i.e., low coefficientof thermal expansion and high corrosion resistance.

The inventors of the present invention carried out extensive research toovercome the foregoing problems. Consequently, the inventors found thatwhen a raw material powder comprising titanium dioxide and alumina issintered in the presence of a specific alkali feldspar and at least onecomponent selected from the group consisting of a Mg-containing oxidewith spinel structure, MgCO₃ and MgO, a sintered body of aluminumtitanate with greatly improved mechanical strength, resistance tothermal decomposition and high refractoriness can be obtained withoutlosing low thermal expansion inherent in aluminum titanate due to thesynergistic effect of the Mg-containing component and alkali feldspar.The present invention was accomplished on the basis of this finding.

Specifically, the present invention provides the raw materialcomposition for preparing a sintered body of aluminum titanate, processfor preparing a sintered body of aluminum titanate and sintered body ofaluminum titanate described below.

1. A raw material composition for preparing a sintered body of aluminumtitanate, the composition comprising:

(i) 100 parts by weight of a mixture comprising 40 to 50 mol% of TiO₂and 60 to 50 mol% of Al₂O₃,

(ii) 1 to 10 parts by weight of alkali feldspar represented by theformula: (Na_(x)K_(l−x))AlSi₃O₈ (0≦x≦1), and

(iii) 1 to 10 parts by weight of at least one Mg-containing componentselected from the group consisting of a Mg-containing oxide with spinelstructure, MgCO₃ and MgO.

2. The raw material composition for preparing a sintered body ofaluminum titanate according to item 1, wherein the alkali feldspar hassuch a composition that x in the formula: (Na_(x)K_(l−x))AlSi₃O₈ is inthe range of 0.1≦x≦1.

3. The raw material composition for preparing a sintered body ofaluminum titanate according to item 1 or 2, wherein the molar ratio ofSi in the alkali feldspar to Mg in the Mg-containing component is in therange of Si:Mg=0.9:1 to 1.1:1.

4. A process for preparing a sintered body of aluminum titanate, theprocess comprising sintering a formed product at a temperature of 1300to 1700° C.

the formed product being prepared from a raw material composition forpreparing a sintered body of aluminum titanate comprising:

(i) 100 parts by weight of a mixture comprising 40 to 50 mol% of TiO₂and 60 to 50 mol% of Al₂O₃, ps (ii) 1 to 10 parts by weight of an alkalifeldspar represented by the formula: (Na_(x)K_(l−x))AlSi₃O₈ (0≦x≦1), and

(iii) 1 to 10 parts by weight of at least one Mg-containing componentselected from the group consisting of a Mg-containing oxide with spinelstructure, MgCO₃ and MgO.

5. A sintered body of aluminum titanate which is obtainable by theprocess of item 4.

The process for preparing a sintered body of aluminum titanate of thepresent invention is a process in which a composition prepared byblending a mixture comprising TiO₂ and Al₂O₃ with an alkali feldsparrepresented by the formula: (Na_(x)K_(l−x))AlSi₃O₈ (0≦x≦1) and at leastone Mg-containing component selected from the group consisting of aMg-containing oxide having spinel structure, MgCO₃ and MgO is used as araw material; and a formed product prepared from this composition issintered at a temperature of 1300 to 1700° C.

TiO₂ and Al₂O₃ used as the raw materials are not particularly limitedinsofar as they are substances from which aluminum titanate can besynthesized by sintering. Usually, they may be suitably selected fromraw materials for producing various ceramics such as alumina ceramics,titania ceramics, aluminum titanate ceramics and so on.

The mixing proportion of TiO₂ and Al₂O₃ may be in a range of 40 to 50mol% of TiO₂ and 60 to 50 mol% of Al₂O₃, preferably 45 to 50 mol% ofTiO₂ and 55 to 50 mol% of Al₂O₃. In particular, adjusting the molarratio of Al₂O₃/TiO₂ to 1 or higher within the mixing proportionmentioned above enables preventing coexistence of a liquid phase.

The alkali feldspar used as an additive is represented by the formula:(Na_(x)K_(l−x))AlSi₃O₈, in which 0≦x≦1. In particular, in theabove-mentioned formula, it is preferable that x is in the range of0.1≦x≦1, and is more preferable that x is in the range of 0.15≦x≦0.85.The alkali feldspar having such a value range of x has a low meltingpoint, and thus is particularly effective for promoting sintering ofaluminum titanate.

The amount of the alkali feldspar used may be about 1 to 10 parts byweight, preferably about 3 to 4 parts by weight, per 100 parts by weightof the total amount of TiO₂ and Al₂O₃.

In the present invention, the Mg-containing oxide with spinel structure,MgCO₃ and MgO may be used singly or in combination of two or more kinds.Among these, examples of usable Mg-containing oxides with spinelstructure include MgAl₂O₄, MgTi₂O₄ and the like. Natural minerals withspinel structure may be used as such oxides. Spinel oxides prepared bysintering a raw material comprising MgO and Al₂O₃ or raw materialcomprising MgO and TiO₂, etc. may be also used. In the presentinvention, two or more different kinds of oxides with spinel structuremay be used in combination.

At least one Mg-containing component selected from the group consistingof a Mg-containing oxide with spinel structure, MgCO₃ and MgO may beused in an amount of about 1 to 10 parts by weight, preferably about 3to 6 parts by weight, per 100 parts by weight of the total amount ofTiO₂ and Al₂O₃.

In the process of the present invention, the molar ratio of Si in thealkali feldspar to Mg in the Mg-containing component is preferably inthe range of Si:Mg=about 0.9:1 to about 1.1:1, more preferably in therange of Si:Mg=about 0.95:1 to about 1.05:1.

According to the process of the present invention, a sintered body ofaluminum titanate with high mechanical strength, resistance to thermaldecomposition and high refractoriness can be provided by mixing theabove-mentioned Mg-containing component and alkali feldspar as additiveswith the mixture comprising TiO₂ and Al₂O₃, forming this mixture into adesired shapes and sintering the same.

The reason why the sintered body with high mechanical strength andresistance to thermal decomposition can be provided by the process ofthe present invention is undetermined, but is presumably as follows:

When aluminum titanate is synthesized by sintering, Si in the alkalifeldspar dissolves into the crystal lattice and replaces Al. Since Sihas a smaller ion radius than Al, the bond length with neighboringoxygen atoms is shortened. The obtained crystal will therefore have asmaller lattice constant than pure aluminum titanate. Accordingly, theresulting sintered body will have a stable crystal structure, improvedmechanical strength and very high thermal stability, leading to greatlyimproved refractoriness.

Further, the use of the Mg-containing component as an additive enablesobtaining compact sintered body. Therefore, it is possible to form asintered body having much higher mechanical strength than pure aluminumtitanate.

According to the process of the present invention, since the alkalifeldspar and the Mg-containing component having such effects are used asadditives in combination, it is presumed that Si contained in the alkalifeldspar and Mg contained in the Mg-containing component replace mainlythe Al sites in aluminum titanate. On the other hand, when each of thesecomponents is added singly, the Al sites, which originally keep theirelectrical charge balance by being trivalent, are replaced by either adivalent (Mg) or tetravalent (Si) element and thus the resultingsintered body needs to keep electrical balance. Therefore, in case whereMg is added, to keep the electrical charge balance, oxygen seems to beejected from the system to cause oxygen deficient in the sintered body.In case of adding tetravalent Si, Ti, which is originally tetravalent,is assumedly reduced to be trivalent to keep the electrical chargebalance. In the present invention, presumably, the electrical chargebalance can be kept by adding the alkali feldspar and Mg-containingcomponent in combination because Mg has a charge number smaller than Alby 1 and Si has a charge number larger than Al by 1, whereby theseelements can dissolve into the sintered body without affecting the otherelements constituting the sintered body. Particularly, when the amountsof the two additives are approximately equimolar, it is expected thatthe additives can exist more stably, compared to the case where they areadded singly. For these reasons, it is presumed that a mechanicalstrength of the sintered body is greatly improved by synergistic effectbetween the two additives, compared to the case where they are usedsingly, whereby the sintered body of aluminum titanate having highmechanical strength exceeding practically usable level and very highrefractoriness due to greatly enhanced resistance to thermaldecomposition is formed without losing low thermal expansion that isinherent in aluminum titanate.

The raw material mixture comprising TiO₂, Al₂O₃, alkali feldspar andMg-containing component may be sufficiently mixed, pulverized to asuitable particle size and then formed into a desired shape.

The process of mixing and pulverizing the raw material mixture is notparticularly limited, and may be any known process, e.g., mixing andpulverizing by using a ball mill, stirred media mill, etc.

The degree of pulverization of the raw material mixture is not critical.Usually, the material is pulverized to a particle size of about 1 μm orless, preferably to as small particle size as possible, as long assecondary particles are not formed.

A forming aid may be added to the raw material mixture, if necessary.The forming aid may be selected from substances which have beenheretofore used depending on the forming method.

Examples of such a forming aid include polyvinyl alcohol, microwaxemulsion, carboxymethyl cellulose and like binders, stearic acidemulsion and like mold releasing agents, n-octyl alcohol, octylphenoxyethanol and like antifoaming agents, diethylamine, triethylamine andlike deflocculants, etc.

The amount of the forming aid used is not critical, and may be suitablyselected within the range of the amount heretofore used for the formingaids depending on the forming method. For example, as a forming aid forslip casting, it is possible to use a binder in an amount of about 0.2to 0.6 parts by weight, a deflocculant in an amount of about 0.5 to 1.5parts by weight, a releasing agent (solid amount) in an amount of about0.2 to 0.7 parts by weight, and an antifoaming agent in an amount ofabout 0.5 to 1.5 parts by weight, per 100 parts by weight the totalamount of TiO₂ and Al₂O₃.

The method of forming the raw material mixture is not particularlylimited and may be suitably selected from known forming methods such aspress molding, sheet casting, slip casting, extrusion molding, injectionmolding, CIP molding, etc.

The sintering temperature may be about 1300 to 1700° C., preferablyabout 1350 to 1450° C.

The atmosphere for sintering is not particularly limited, and may be anyof an oxygen-containing atmosphere such as air, a reducing atmosphereand an inert atmosphere, which are heretofore employed.

The sintering time is not particularly limited, and may be such that theproduct is sufficiently sintered depending on the shape of the formedproduct, etc. Usually, the sintering is conducted for about 1 to 10hours while maintaining the temperature range mentioned above. Theheating rate and cooling rate in sintering are not particularly limited,and may be suitably selected so that no cracks are formed in thesintered body.

The sintered body obtained by the process of the present invention hasthe features mentioned above; for example, high mechanical strength andlow coefficient of thermal expansion at the same time. Further, thesintered body has excellent resistance to decomposition due to thestable crystal structure, and a high value of refractoriness.Accordingly, decomposition reaction of aluminum titanate is inhibitedand the product can be stably used even at high temperatures such as afew hundred to about 1600° C. As for bending strength, a very highbending strength of over about 90 MPa, which is about 6 times higherthan those of known sintered bodies of aluminum titanate, can beattained. In addition, according to the process of the presentinvention, sintering without the formation of cracks is possible so thatthe resulting sintered body becomes compact and has high resistance tothermal shock.

The sintered body prepared by the process of the present invention showsvery high non-wettability and corrosion resistance against molten metal.As a result, it exhibits such excellent erosion resistance against flowof molten metal that could never be expected for known materials.

The sintered body of aluminum titanate of the present invention,utilizing its excellent features mentioned above, can be used forvarious applications, for example, containers for high-melting pointmetals such as crucibles for melting metals, ladles and gutters;components for high-temperature portions of aircraft jet engines; jetnozzles; components for high-temperature portions of various internalcombustion engines such as glow plugs, cylinders and piston head;insulating and shielding plates for outer walls of spacecrafts, etc.Furthermore, it can be effectively used as a surface plate for printingprocessing in LSI manufacturing processes, etc., utilizing its lowexpansibility.

As can be seen from the above, the sintered body of aluminum titanateobtained by the producing process of the present invention has highmechanical strength and resistance to thermal shock, while maintainingthe inherent low expansion coefficient of aluminum titanate. Inaddition, the sintered body of aluminum titanate has excellentresistance to decomposition, exhibits a high value of refractoriness,and can be stably used at high temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the change of residual aluminum titanatepercentage over time in the sintered body of aluminum titanate accordingto the present invention placed in the atmosphere at 1000° C.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below in more details with referenceto the following examples.

Example 1

To 100 parts by weight of a mixture comprising 43.9% by weight (50 mol%) of titanium oxide in anatase form and 56.1% by weight (50 mol %) ofsinterable α-alumina were added 4 parts by weight of the alkali feldsparrepresented by the formula: (Na_(0.6)K_(0.4))AlSi₃O₈, 6 parts by weightof the spinel represented by the formula: MgAl₂O₄, 0.25 parts by weightof polyvinyl alcohol as a binder, 1 part by weight of diethylamine as adeflocculant and 0.5 parts by weight of polypropylene glycol as anantifoaming agent. The mixture was mixed using a ball mill for 3 hoursand then dried using a dryer at 120° C. for 12 hours or more, giving araw material powder.

The resulting raw material powder was pulverized to about 150 mesh andpressed under a pressure of 60 MPa, giving a molded product measuring100 mm×100 mm×10 mm.

This molded product was sintered according to the heating patterndescribed below in the atmosphere and thereafter left to cool, giving asintered body of aluminum titanate.

(Heating pattern)

from 0 to 180° C. over 6 hours

maintained at 180° C. for 4 hours (water evaporation)

from 180 to 340° C. over 4 hours

maintained at 340° C. for 4 hours (organic binder combustion)

from 340 to 700° C. over 4 hours

maintained at 700° C. for 2 hours (residual carbon combustion)

from 700 to 1400° C. over 4 hours

maintained at 1400° C. for 4 hours

A 5 mm×5 mm×20 mm sample was cut from the resulting sintered body. Thesurface of the sample was polished, and the coefficient of thermalexpansion of the sample was determined at the heating rate of 20°C./min. The results are shown in Table 1 below. TABLE 1 Percentage ofAverage thermal expansion coefficient of Temperature (ΔL/L) thermalexpansion (° C.) % (×10⁻⁷/K) 126 −0.016 −15.55 226 −0.029 −14.26 326−0.036 −12.13 426 −0.040 −10.10 526 −0.039 −7.87 626 −0.033 −5.47 726−0.014 −2.07 826 0.012 1.55 926 0.035 3.85

As can be seen from the results shown above, the sintered body obtainedby the process of the present invention has a low coefficient of thermalexpansion and maintains the inherent low thermal expansibility ofaluminum titanate.

Example 2

A sintered body of aluminum titanate was obtained in the same manner asin Example 1 using the same raw material as that used in Example 1except that the heating pattern in sintering was as described below.

(Heating pattern)

from 0 to 180° C. over 6 hours

maintained at 180° C. for 4 hours (water evaporation)

from 180 to 340° C. over 4 hours

maintained at 340° C. for 4 hours (organic binder combustion)

from 340 to 700° C. over 4 hours

maintained at 700° C. for 2 hours (residual carbon combustion)

from 700 to 1350° C. over 4 hours

maintained at 1350° C. for 4 hours

A 3 mm×4 mm×40 mm sample was cut from the resulting sintered body ofaluminum titanate. The surface of the sample was polished, and thesample was tested for its three-point bending strength.

For comparison, two control examples were prepared: a sintered body(Comparative Example 1) obtained by using the raw material having thesame formulation as that used in Example 1 and sintering in the samemanner as in Example 2 (sintering temperature: 1350° C.) except that nospinel was used and 4 parts by weight of alkali feldspar was used as anonly additive; and a sintered body (Comparative Example 2) obtained byusing the raw material having the same formulation as that used inExample 1 described above and sintering under the same conditions as inExample 2 (sintering temperature: 1350° C.) except that no alkalifeldspar was used and 6 parts by weight of spinel was used as an onlyadditive. These sintered bodies were tested for their three-pointbending strength in the same manner. The results are shown in Table 2below. TABLE 2 Three point bending Sample name strength (MPa) Example 2(feldspar 85.7 and spinel added) Comparative Example 1 56.0 (feldsparadded) Comparative Example 2 30.3 (spinel added)

As can be seen from the results shown above, the sintered body ofaluminum titanate of Example 2 obtained by adding alkali feldspar andspinel at the same time has higher mechanical strength than the sinteredbodies of aluminum titanate of Comparative Example 1 and ComparativeExample 2 obtained by adding either alkali feldspar or spinel.

In addition, a sample measuring 10 mm×10 mm×10 mm was cut from each ofthe sintered bodies of aluminum titanate of Example 2, ComparativeExample 1 obtained by adding alkali feldspar only and ComparativeExample 2 obtained by adding spinel only. The samples were placed in theatmosphere at 1000° C., and the change of the residual aluminum titanatepercentage over time was determined by X-ray diffraction method. Theresidual aluminum titanate percentage was calculated by measuring thediffraction intensity of the (110) and (101) faces of rutile, anddetermining the amount of rutile from the sum of their areas becausealuminum titanate decomposes into alumina and rutile.

Further, the sintered body obtained by using the raw material having thesame formulation as that used in Example 1 except that neither spinelnor alkali feldspar was added and sintering under the same conditions asin Example 1 (sintering temperature: 1400° C.) was tested for the changeof the residual aluminum titanate percentage over time in the samemanner. The results are shown as a graph in FIG. 1

As can be seen from FIG. 1, the sintered body of aluminum titanate ofExample 2 was hardly decomposed into TiO₂ and Al₂O₃ when it was left ata high temperature for a long period. This demonstrates that it hasexcellent resistance to thermal decomposition.

1. A raw material composition for preparing a sintered body of aluminum titanate, the composition comprising: (i) 100 parts by weight of a mixture comprising 40 to 50 mol % of TiO₂ and 60 to 50 mol % of Al₂O₃, (ii) 1 to 10 parts by weight of alkali feldspar represented by the formula: (Na_(x)K_(1−x))AlSi₃O₈ (0≦x≦1), and (iii) 1 to 10 parts by weight of at least one Mg-containing component selected from the group consisting of a Mg-containing oxide with spinel structure, MgCO₃ and MgO.
 2. The raw material composition for preparing a sintered body of aluminum titanate according to claim 1, wherein the alkali feldspar has such a composition that x in the formula: (Na_(x)K_(1−x))AlSi₃O₈ is in the range of 0.1≦x≦1.
 3. The raw material composition for preparing a sintered body of aluminum titanate according to claim 1 or 2, wherein the molar ratio of Si in the alkali feldspar to Mg in the Mg-containing component is in the range of Si:Mg=0.9:1 to 1.1:1.
 4. A process for preparing a sintered body of aluminum titanate, the process comprising sintering a formed product at a temperature of 1300 to 1700° C. the formed product being prepared from a raw material composition for preparing a sintered body of aluminum titanate comprising: (i) 100 parts by weight of a mixture comprising 40 to 50 mol % of TiO₂ and 60 to 50 mol % of Al₂O₃, (ii) 1 to 10 parts by weight of an alkali feldspar represented by the formula: (Na_(x)K_(1−x))AlSi₃O₈ (0≦x≦1), and (iii) 1 to 10 parts by weight of at least one Mg-containing component selected from the group consisting of a Mg-containing oxide with spinel structure, MgCO₃ and MgO.
 5. A sintered body of aluminum titanate which is obtainable by the process of claim
 4. 